This invention relates to a video disc player having a concentric or spiral recording track and more particularly to a video disc player with a skip mechanism and an arm servo system which can realize a stable regular mode playback or reproduction and a special playback in a mode such as a search mode, as fast forward mode or a fast reverse mode.
In a video disc player with a spiral recording track serving as guide grooves, a scratched or defective groove due to faulty manufacturing or ensuing careless handling and use gives rise to an undesired condition called "locked groove". In order for a signal sensing element or a stylus used in a very high-density disc (VHD) system or a capacitance electronic disc (CED) system to escape from the locked groove, there is required a skip mechanism which can forcibly move the signal sensing element or stylus from the locked groove to a normal groove.
To explain a conventional skip function, reference should be made to FIG. 1 which shows a relative positional relation between a stylus tip 22A for disc signal detection and a grooved disc 10. The stylus is typically carried on a pickup arm or cantilever which is supported movably in both vertical and lateral directions. In FIG. 1, while the stylus postures as shown at solid lines (I) and (III) during regular reproduction, it postures as shown by a chained line (II) during its shifting from (I) position to (III) position, namely, during the skip operation.
As will be understood easily, it is necessary to apply to the signal sensing element, a stylus horizontal drive force F.sub.V which is vertical to the direction of the groove and torque F.sub.t about the center of gravity of the stylus tip 22A during the skip operation as shown at arrows in FIG. 1.
According to an experiment conducted by the present inventors, under the application of only the horizontal drive force F.sub.V, the stylus tip 22A sometimes shifts excessively across several of tens of grooves, for example. An excessive shifting exerts a corresponding reaction on the stylus tip 22A and the stylus tip 22A is forced to finally shift in a direction opposite to an intended direction.
Under the application of only the torque F.sub.t, the stylus tip 22A merely rotates and the probability of skip shifting is decreased considerably.
Illustrated in FIG. 2 is a conventional known technique for providing the skipper function. In FIG. 2, there are seen skip coils 51a and 51b, a skip magnet 53, a cantilever 54, a pickup stylus 55, and a video disc 56.
The pickup stylus 55 is supported by the cantilever 54 and scans along a signal groove in the video disc 56 a recording signal recorded in the signal groove. The cantilever 54 is pivoted at its center of gravity, and the skip magnet 53 is fixedly secured to the cantilever 54 with its S pole, for example, located close to the cantilever and its N pole remote therefrom.
The skip coils 51a and 51b are disposed on both sides of the skip magnet 53 as shown in FIG. 2. The skip magnet 53, cantilever 54 and pickup stylus 55 are offset from a center line 52 between the skip coils 51a and 51b toward the periphery of the video disc 56 i.e. in the forward direction of the spiral groove.
Normally, while scanning and reproducing a recording signal on the video disc 56, the pickup stylus 55 moves toward the periphery of the video disc 56, namely, in the forward direction 57a of the pickup stylus 55. The moving rate is given by equation (1): EQU 2.66 .mu.m.times.7.5 rps.div.20 .mu.m/sec (1)
where 2.66 .mu.m is a track groove pitch on the video disc pursuant to the CED system standard and 7.5 rps corresponds to a disc revolution of 450 rpm.
In the event that the pickup stylus 55 faces a defective signal groove in the video disc 56 and is prevented from moving in the forward direction 57a, resulting in a phenomenon called "locked groove" in which the same signal is reproduced repeatedly, the pickup stylus 55 is required to forcibly jump to another groove in the forward direction 57a of the pickup stylus 55.
Conventionally, upon occurrence of the locked groove, currents are fed to the skip coils 51a and 51b so that one skip coil 51a opposing the skip magnet 53 may produce an N pole, for example, and the other skip coil 51b may produce an S pole as shown in FIG. 2.
With the magnetic poles thus produced by the skip coils 51a and 51b, the N pole of the skip coil 51a exerts attractive force 59 on the S pole of skip magnet 53 close to the cantilever 54 in an obliquely upward direction and repulsive force 58 on the N pole of skip magnet 53 remote from the cantilever 54 in substantially the horizontal direction toward the S pole of the skip coil 51b.
As a result, the pickup stylus 55 is slightly rotated about the pivotal fulcrum of the cantilever 54 and raised so as to jump to another groove in the forward direction, thereby escaping from the locked groove.
Unless the pickup stylus 55 is offset from the center line 52 toward the periphery of the video disc in contrast to the precedence, the force 59 for obliquely raising the S pole of skip magnet 53 close to the cantilever 54 becomes insufficient and the skip operation for causing the pickup stylus to forcibly jump over the signal groove in the video disc 56 becomes almost impossible.
Based on the skip operation, a fast reproduction is carried out as will be described below. In a video disc player of the CED system, 8-field pictures are recorded for one revolution. When the skip operation is effected by picking up a vertical synchronizing signal added between picture fields, it is possible to obtain an 8+1=9 times fast forward reproduction and an 8-1=7 times fast reverse reproduction.
The skip operation effected by picking up the vertical synchronizing signal is effective to suppress display of noise due to the skip operation (background noise) on the playback screen.
The 9 times fast forward reproduction can be accomplished by sequential escape from the locked groove. But, to accomplish the 7 times fast reverse reproduction, the skip coils 51a and 51b must produce opposite magnetic poles to those of FIG. 2 and must be adjusted so as to be offset from the center line 52 between the skip coils 51a and 51b in a direction opposite to the forward direction 57a shown in FIG. 2. Such an adjustment, when taking the eccentricity of the video disc into consideration, is difficult to achieve and practically, the fast reverse reproduction is almost impossible.
In the video disc player, the signal sensing element such as for example the stylus is supported by a cartridge such as an arm. The signal sensing element may also be supported in a suspension fashion as in a two-dimension tracker of the VHD system or a three-dimension actuator of the optical disc player. When the stylus is moved for reproduction, the relative position of the stylus to the pickup arm or cartridge must be held constant. To this end, an arm servo mechanism is provided which detects the relative position between the stylus and the cartridge and brings a detected value into coincidence with a preset value or a reference value.
The arm servo mechanism itself is disclosed in the background of the invention in the above-mentioned Furuichi et al patent and will not be detailed herein.
With the conventional skip mechanism and the arm servo mechanism, simultaneous production of both the torque F.sub.t and the horizontal drive force F.sub.V which is vertical to the groove was difficult to achieve. In addition, it was practically impossible to optimize the torque and the horizontal drive force in accordance with condition of the locked groove, in other words, it is difficult to attain the sequential movement of the stylus over grooves upon the special reproduction and the jump of the stylus over a desired number of grooves in the event of occurrence of the locked groove.